Saturn's Icy Satellites

Except for Titan, Saturn's moons are usually referred to as icy satellites because of their rock-hard surfaces consisting mainly of water ice. Scientists have long believed that the icy satellites were too small to have enough gravity to hold on to a substantial atmosphere. For many years, scientists also thought Saturn's icy satellites were geologically dead. They assumed that any internal source of heat the satellites might have was too limited to affect the surface in any way. (On Earth, radioactive decay in the core produces heat that fuels volcanic activity and powers the movement of the tectonic plates that make up Earth's outer surface.)

Images from the Voyager probes picked up clues that the moon Enceladus was ejecting material that became part of the scattered E ring, Saturn's outermost ring. In March 2005, Cassini discovered that Enceladus, which has a diameter of only about 500 kilometers (300 miles), actually has an atmosphere. The spacecraft's cosmic dust analyzer revealed that this atmosphere consists of tiny pieces of water ice only about 1 micron (0.0000394 inch) in diameter. Because of Enceladus's light gravitational force, however, these particles float away over time, becoming part of the E ring. Thus, scientists have speculated, Enceladus must have geysers or volcanoes or another means of replenishing its atmosphere.

Cassini also gave scientists their first close look at Phoebe, one of Saturn's darkest moons. Voyager 2 saw Phoebe in 1981 from a distance of 2.2 million kilometers (1.4 million miles). The Cassini spacecraft, however, flew within 2,060 kilometers (1,285 miles) of Phoebe on June 11, 2004. It was Cassini's only opportunity to fly that close to Phoebe. But that one flyby provided more information about Phoebe than scientists had learned since the moon's discovery in 1898.

Cassini's images of Phoebe revealed a battered world scarred with impact craters and littered with landslides and small boulders. In the bottom of some of the craters lay boulders hundreds of meters (thousands of feet) wide. Cassini's visual and infrared mapping spectrometer confirmed Earth-based conclusions that Phoebe's dark surface consists of water ice darkened by a covering of carbon compounds and other material. (A spectrometer is an instrument that spreads out light and other types of electromagnetic waves into a spectrum and displays it for study.)

Scientists were elated to see images showing bright streaks on the walls of Phoebe's largest craters and bright rays originating from its small craters. They believe these features were created as meteors or other objects broke through Phoebe's dark crust during bombardments, exposing the moon's deeper layers. Cassini's infrared and ultraviolet instruments also discovered that the bright streaks are water ice with some carbon dioxide ice.

We now know that Phoebe is a mixture of water ice, rock, carbon dioxide, and primitive organic (carbon-containing) compounds, much like Pluto and Neptune's moon Triton. Scientists believe that icy bodies like Phoebe were plentiful in the outer regions of the solar system about 4.6 billion years ago and were the building blocks of the outer planets—Jupiter, Saturn, Uranus, and Neptune. As these planets formed, gravitational forces ejected most of the leftover blocks to orbits outside the solar system. Phoebe became trapped in Saturn's orbit instead. As a result, we have the first detailed images of such a primitive object.

Saturn's oddest moon, Iapetus, also revealed some of its secrets to Cassini. One side of this moon is bright, while the other side is dark. Voyagers 1 and 2 flew by Iapetus, but the dark side appeared only as an inky blackness in the probes' cameras. Cassini's more sophisticated cameras returned images of both sides. The dark-side images showed startling surface features, including a long, narrow, soaring ridge that lies almost exactly on Iapetus's equator. The ridge is about 1,300 kilometers (800 miles) long, with some mountains at least 20 kilometers (12 miles) high, about three times higher than Earth's Mount Everest. The ridge narrows as it rises, from a base that is perhaps 20 kilometers (12 miles) wide to peaks only 2 kilometers (1.2 miles) across.

At the boundary between the bright and dark materials, the images show craters with bright walls on one side and dark walls on the other. The colors most likely relate to the material making up the moon. The dark material is probably organic compounds. The light-colored material is probably water ice and carbon dioxide ice, which are very bright.

Also at the boundary between the bright and dark material, the images show feathery-looking black streaks. Scientists disagree whether this dark material spews from the interior of Iapetus or rains down onto Iapetus from elsewhere, perhaps from nearby Phoebe. From looking at the meteorites that fall to Earth, we know that carbonaceous chondrites, the most primitive type of meteorite, are rich in dark materials like those from which the sun and planets formed. Perhaps at one time, Iapetus went through a cloud of this material. Scientists hope that Cassini's next flyby of Iapetus in 2007 will uncover more clues to this mystery.